Ay, well, I've taken a few art classes in my time, and there was always a piece of advice my teacher would give. He told us that yes, copying exactly from a photograph could be considered "stealing". The reason being is that the photographer put all the work into creating a composition and finding an object to photograph and therefore the photo still is their sole work. This is why you can see some photographs copyrighted. Now, you can't really copyright nature, so if you take the photo yourself or go out and observe from nature that's fine!
In order to avoid this "stealing", my teacher would encourage us to use the photo, but instead make it our own. So for example, if you were referencing a dog photo, then add something of your own, or change something or do something else to make the work your own. Of course, if you are only studying this photo for anatomy or pose, then this is also fine, as you are going to change the pose/anatomy into your character. Hope this helps ya out a bit. And when in doubt, always ask the artist/photographer for permission first! c:
